In the development of coatings, the viscosity of the initial formulation as well as the curing behaviour of the batch is affected by, for example, variation of catalysts, binders, curers or solvents. For processing, however, for example in a spray gun, it is necessary for the viscosity to vary in a particular range.
For the processing of systems which cure, for example in the field of two-component (2C) coatings, this gives rise to a time limitation, the so-called “pot life”, after the expiration of which the coating is no longer processable since its viscosity has increased too much. The pot life in this case reflects the period of time between mixing of the binder and curer and the time when the material is no longer processable. In the development of new coating systems, it is therefore necessary for them to have a controlled pot life.
For the development of coatings using methods from combinatorial chemistry, large quantities of different substances to be tested are encountered within a short time, the available quantity of substance generally being limited to a few grams. In technical coating development, the method according to DIN EN ISO 9514 is customarily used for determining the pot life. A special vessel with a fixed volume (standard flow beaker) is filled with a substance, which then flows out through a hole in the bottom of the vessel. The flow time is measured. The pot life is determined by the increase in the flow time by a particular factor, generally twice the initial value. Flow beakers are calibrated using newtonian liquids of known viscosity.
Commercially available viscosimeters operate according to various functional principles, as described for example in Brock/Groteklaes/Mischke, Lehrbuch der Lacktechnologie, Vincentz Verlag Hanover 1998.
Capillary viscosimeters, for example Ubbelohde viscosimeters, are instruments for determining the kinematic viscosity of liquids.
The liquid is forced at a defined pressure through a capillary of particular length and particular radius. Either the time taken for a particular volume to flow through is measured or, conversely, the volume which flows through during a particular time is measured. The liquid may in this case be forced or sucked through the capillary. For low-viscosity liquids, the operating pressure can be generated simply by gravity with a storage vessel being placed at a raised height. The evaluation is carried out according to the Hagen-Poisseuille law:                                           ⅆ            V                                ⅆ            b                          =                                            π              ·                              r                4                                                    8              ⁢                                                           ⁢              η              ⁢                                                           ⁢              l                                ⁢          Δ          ⁢                                           ⁢                      p            .                                              (        1        )            The falling ball viscosimeter (Höppler viscosimeter) consists of a glass measuring tube, which holds the liquid to be tested and 6 balls. The fall time of the balls between two measuring markers is determined, and the dynamic viscosity of the liquid can be calculated from this via the density difference between the balls and the liquid and an equipment constant.
Another way of determining the rheology of substances is provided by the rheometer. In this case, a layer of the liquid is placed, and sheared, between two bodies of defined geometry. Inter alia, the viscosity can be determined from the response of the liquid to shearing. Examples of conventional geometries are plate/plate, plate/tip, plate/spherical shell or concentric cylinders. An advanced method for determining rheological parameters is described, for example, in Ultrasonics 36 (1998), pages 483-490.
Disadvantages with all the described methods are that the instruments only operate serially, the measurements sometimes last a very long time, for example rheometers, and the measuring instruments need to be cleaned after each measurement.
It was an object of the present invention to provide a purely qualitatively operating method for determining viscosities, with which a large number of samples can be evaluated within a short time. The equipment developed for carrying out this method should in this case make it possible to evaluate a fairly large number of samples in parallel and avoid onerous cleaning of the equipment components, in particular the measuring capillaries. The data acquisition should in this case be carried out as automatically as possible.